The present invention relates to a line head and an image forming apparatus incorporating the same.
In general, a toner image former using an electrophotographic method comprises a photosensitive member serving as an image carrier having a photosensitive layer on an outer peripheral face, a charger for uniformly charging the outer peripheral face of the photosensitive member, an exposer for selectively exposing the outer peripheral face charged uniformly by the charger to form an electrostatic latent image, and a developing device for giving a toner serving as a developer to the electrostatic latent image formed by the exposer, thereby forming a visible image (a toner image).
In an image forming apparatus using a tandem method which forms a color image, a plurality of (for example, four) toner image former is provided on an intermediate transfer belt. Some apparatuses are of an intermediate transfer belt type which sequentially transfers toner images formed on a photosensitive member by monochromatic toner image former onto an intermediate transfer belt and superposes toner images having a plurality of colors (for example, yellow, cyan, magenta and black) on the intermediate transfer belt, thereby obtaining a full color image on the intermediate transfer belt.
Moreover, there has been known a color image forming apparatus comprising an image carrier constituted to freely carry an electrostatic latent image thereon, a rotary developing unit and a line head. In the image forming apparatus, the rotary developing unit carries, on a face thereof, toners accommodated in a plurality of toner cartridges and rotates in a predetermined rotating direction, thereby delivering the toners having different colors to an opposed position to the image carrier sequentially. Then, a developing bias is applied between the image carrier and the rotary developing unit to move the toner from the rotary developing unit to the image carrier. By such a processing, the electrostatic latent image is revealed to form a toner image.
There has been known an image forming apparatus using the tandem method or a rotary method in which an LED (light emitting diode) or an organic EL (electroluminescence) element is used as a light emitter in a line head. In some cases, the line head having such a structure is attached with a tilt with respect to a reference position in a primary scanning direction. In FIG. 21, (a) shows a state in which the line head is placed in a reference attachment position which is parallel with the primary scanning direction, (b) shows a state in which the attachment is carried out with a tilt corresponding to several dots in an upward direction in the drawing, and (c) shows a state in which the attachment is carried out with a tilt corresponding to several dots in a downward direction in the drawing.
As described above, when the line head is attached with a tilt with respect to the reference position in the primary scanning direction, a density fluctuation is generated so that the quality of an image is deteriorated. Moreover, there is a problem in that a color deviation is generated and a desirable color reproduction cannot be carried out in the formation of a color image. In order to prevent a deterioration in an image from being caused by the tilt of the line head, various proposals for detecting the tilt of the line head have been made. For example, in Japanese Patent Publication No. 63-278074A, an image signal generator for forming a test pattern image, to be used for a test measurement, on a transfer belt for each color and the test pattern image is detected by a detector.
In the technique described in the Japanese Patent Publication No. 63-278074A, the test pattern is generated and is input from an outside to an image forming apparatus. Moreover, it is necessary to hold the test pattern as stored data in a controller provided in the image forming apparatus. For this reason, a great deal of time and labor is required for creating the test pattern, and furthermore, the structure of the controller in the image forming apparatus is complicated and the tilt of a line head cannot be detected easily.
Japanese Patent Publication No. 3-272873A discloses the correction of a tilt in a case where an EL element of an end face light emission type is used for the line head. In this example, the width of a primary scanning line is divided into a plurality of portions so that the light emission timing is adjusted for each of the divided portions in order to correct the tilt of the line head.
Japanese Patent Publication No. 7-304211A discloses the correction of the tilt of an LED head. In this example, tilt correction data are added to image data for each dot and the corrected data are output to a recording head, thereby correcting the tilt of the LED head.
In the technique described in Japanese Patent Publication No. 3-272823A, a writing operation is always started in the same timing for each light emitter array corresponding to each divided portion in order to simplify the circuit structure and the correction control. Therefore, it is impossible to perform the tilt correction adjustment with respect to an arbitrary number of light emitters. In the technique described in Japanese Patent Publication No. 7-404211A, furthermore, the amount of the correction of a tilt is based on a unit of a primary scanning line. Therefore, there is a problem in that a fine control cannot be carried out.